Soldering flux is normally applied to electrical assemblies, such as printed circuit boards, before they are soldered in order to clean the electrical conductors and improve solder flow. In a high production environment, the printed circuit boards (PCB's) are passed over a foam of liquid flux bubbles which rise through the top of a chimney flue as overflowing foam. The face of the PCB is passed through and into intimate contact with the overflowing foam. The size of the foam bubbles determines, to a large extent, whether the proper amount of flux is applied to the PCB. If either too much or not enough flux is applied to the PCB, the quality of resulting solder joints may be affected.
It is believed that as a result of surface tension and on the overflowing foam, the rising bubbles in the foam have a tendency to combine into larger bubbles at the face of the PCB. the downwardly exerted pressure of the PCB. Inasmuch as there is more air in the larger bubbles, they tend to burst more readily, causing uneven covering of flux on the PCB. Uneven flux covering, in turn, results in incomplete soldering or excessive "solder voids". These solder voids must be filled by hand soldering techniques after the PCB is removed from the automatic soldering line or "solderpot".
It would therefore be desirable to eliminate the presence of large flux bubbles at the face of the PCB in order to avoid excessive solder voids. The present invention is directed toward achieving this objective.